Wellsite System and Method for Multiple Carrier Frequency, Half Duplex Cable Telemetry

ABSTRACT

Methods and systems for multiple carrier frequency, half duplex cable telemetry for a wellsite. The methods involve generating a first type of bi-directional message in a first propagation mode, generating a second type of bi-directional message in the first propagation mode and in a second propagation mode, transmitting over a cable operatively coupling a surface modem and a downhole modem the first and second types of bi-directional message sequentially in a plurality of time periods across a single frequency bandwidth, and separating each of the first and second types of bi-directional message from a most subsequently transmitted one of the first and second types of bi-directional message by a quiet time sample during which no message is transmitted.

BACKGROUND

The following descriptions and examples are not admitted to be prior artby virtue of their inclusion in this section.

Hydrocarbon fluids, such as oil and natural gas, may be obtained from asubterranean geologic formation, referred to as a reservoir, by drillinga well that penetrates a hydrocarbon-bearing formation. A variety ofdownhole tools may be used in various areas of oil and natural gasservices. In some cases, downhole tools may be used in a well forsurveying, drilling, and production of hydrocarbons. The downhole toolsmay communicate with the surface via various telemetry systems. In somecases, the downhole tools may comprise one or more individual modules inoperative communication with one another, such as a master module andmultiple slave modules. Examples of communication systems are providedin U.S. Pat. Nos. 6,628,992, 7,181,515, and 20020178295.

With the increased precision of downhole tools and sensors, relativelyshorter time may be available to send increasingly larger amounts ofdata. In addition to new modules and assemblies being developed fordownhole use on a continuing basis, tool bus systems may facilitatecommunication between older and newer generation modules in order toobtain the maximum service life from existing modules.

Applications of disclosed embodiments of the present disclosure are notlimited to these illustrated examples, different industrial applicationsmay benefit from implementations of the following disclosure.

SUMMARY

In at least one aspect, the disclosure relates to a method for multiplecarrier frequency, half duplex cable telemetry. The method can includegenerating a first type of bi-directional message in a first propagationmode. The method can include generating a second type of bi-directionalmessage in the first propagation mode and a second propagation mode. Themethod can include transmitting the first and second types ofbi-directional messages over a cable operatively coupling a surfacemodem and a downhole modem sequentially in a plurality of time periodsacross a single frequency bandwidth. The method can include separatingeach of the first and second types of bi-directional messages from themost subsequently transmitted message by a quiet time sample duringwhich no message is transmitted.

In at least one aspect, the disclosure relates to a system for multiplecarrier frequency, half duplex cable telemetry. The system can include asurface modem that generates one or more downlink messages in a firstpropagation mode. The system can include one or more downhole tools thatobtain measurements relating to one of borehole characteristics andformation characteristics, the downhole tools coupled by a toolbus to adownhole modem. The downhole modem generates one or more uplink messagesin the first propagation mode and a second propagation mode. The systemcan include a cable electrically coupling the surface modem and thedownhole modem. The one or more downlink messages and the one or moreuplink messages are transmitted over the cable sequentially in aplurality of time periods across a single frequency bandwidth, each ofthe downlink and uplink messages separated from the most subsequentlytransmitted message by a quiet time sample during which no message istransmitted.

In at least one aspect, the disclosure relates to a wellsite system formultiple carrier frequency, half duplex cable telemetry. The wellsitesystem can include a surface acquisition unit including a surface modemthat generates one or more downlink messages in a first propagationmode. The wellsite system can include a downhole modem that generatesone or more uplink messages in the first propagation mode and a secondpropagation mode. The wellsite system can include a downhole tool stringincluding one or more downhole sensing tools that obtain measurementsrelating to one of borehole characteristics and formationcharacteristics. The downhole tool string may be operatively coupled tothe downhole modem via a toolbus. The wellsite system can include acable electrically coupling the surface modem and the downhole modem.The one or more downlink messages and the one or more uplink messagesmay be transmitted sequentially in a plurality of time periods across asingle frequency bandwidth, each of the downlink and uplink messagesseparated from the most subsequently transmitted message by a quiet timesample during which no message is transmitted.

This summary is provided to introduce a selection of concepts that arefurther described below in the detailed description. This summary is notintended to identify key or essential features of the claimed subjectmatter, nor is it intended to be used as an aid in limiting the scope ofthe claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of systems, apparatuses, and methods for multi-carrierfrequency, half duplex cable telemetry are described with reference tothe following figures. Like numbers are used throughout the figures toreference like features and components.

FIG. 1 is a schematic representation illustrating a wellsite with aborehole traversing a subsurface formation and having a system formultiple carrier frequency, half duplex cable telemetry in accordancewith an embodiment of the present disclosure.

FIG. 2 shows a block diagram illustrating an example system for multiplecarrier frequency, half duplex cable telemetry in accordance with anembodiment of the present disclosure.

FIGS. 3-1 through 3-3 are schematic diagrams illustrating end views of awireline heptacable in accordance with an embodiment of the presentdisclosure.

FIG. 4 is a graph illustrating uplink and downlink spectrum allocationtransmitted in a multi-carrier frequency, full duplex telemetryarchitecture.

FIG. 5 is a graph illustrating uplink and downlink spectrum allocationtransmitted in a multi-carrier frequency, half duplex telemetryarchitecture in accordance with an embodiment of the present disclosure.

FIG. 6 shows a flow chart illustrating a method of multi-carrierfrequency, half duplex cable telemetry in accordance with at least oneembodiment of the present disclosure.

FIG. 7 is a schematic diagram illustrating end views of variouspropagation modes applied in wireline communication along a heptacablein accordance with an embodiment of the present disclosure.

FIG. 8 is a schematic block diagram of a cable telemetry systemillustrating the passage of signals therethrough in accordance with anembodiment of the present disclosure.

FIG. 9 is a schematic diagram of a cable telemetry system illustratinguplink and downlink therethrough in accordance with an embodiment of thepresent disclosure.

FIG. 10 is a schematic diagram illustrating a cumulative signal inaccordance with an embodiment of the present disclosure.

FIG. 11 is a schematic diagram illustrating cumulative cross-talksources on a wireline heptacable in accordance with an embodiment of thepresent disclosure.

FIG. 12 is a schematic diagram illustrating a time-domain diagram ofuplink and downlink frames in a half duplex architecture in accordancewith an embodiment of the present disclosure.

FIGS. 13-1 and 13-2 are graphs illustrating oscilloscope captures ofuplink frames and downlink frame in accordance with an embodiment of thepresent disclosure.

FIGS. 14-1 and 14-2 are graphs illustrating a shortened and anunshortened ECHO impulse response after and before TEQ respectively inaccordance with an embodiment of the present disclosure.

FIG. 15 is a graph illustrating cable delay on guard interval length inaccordance with an embodiment of the present disclosure.

FIG. 16 is a graph illustrating a process to manage superpacket dataflow in accordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION

In the following description, numerous details are set forth to providean understanding of the present disclosure. However, it will beunderstood by those skilled in the art that the present disclosure maybe practiced without these details and that numerous variations ormodifications from the described embodiments are possible.

The disclosure relates to cable telemetry for a wellsite. The cabletelemetry may enable data reception from tools and send data commands todownhole tools via, for example, a wireline heptacable. “Cabletelemetry” refers generally to communication between an uphole modem anda downhole modem over a cable.

Multi-Carrier Frequency, Half Duplex Cable Telemetry Overview

Referring to FIG. 1, an example wireline logging operation isillustrated with respect to the wellsite system 100 employed in awellbore 102 traversing a subsurface formation 104. A downhole telemetrycartridge 110 is connected to a toolstring 116. In a well-loggingoperation, a plurality of tools (e.g., 230, 230′, etc. of FIG. 2) may beconnected in the toolstring 116. The tools of the toolstring 116communicate with the downhole telemetry circuits of downhole telemetrycartridge 110 via a bi-directional electrical interface.

In some embodiments, the tools of the toolstring 116 may be connected tothe telemetry cartridge 110 over a common data bus. In some embodiments,each tool of the toolstring 116 may be individually, directly connectedto the telemetry cartridge 110. In one embodiment, the telemetrycartridge 110 may be a separate unit, which is mechanically andelectrically connected to the tools in the toolstring 116. In oneembodiment, the telemetry cartridge 110 may be integrated into a housingof one of the well-logging tools 116.

The telemetry cartridge 110 is operatively coupled to a wireline cable114. The tools of the toolstring 116, including the telemetry cartridge110, may be lowered into the wellbore 102 on the wireline cable 114.

A surface data acquisition computer 118 is located at the surface end ofthe wireline cable 114. The surface data acquisition computer 118includes or couples to an uphole telemetry unit 112. The dataacquisition computer 118 may provide control of the components in thetoolstring 116 and process and store the data acquired downhole. Theacquisition computer 118 may communicate with the uphole telemetry unit112 via a bi-directional electrical interface.

The uphole telemetry unit 112 may modulate downlink commands from theacquisition computer 118 for transmission down the cable 114 to thetoolstring 116, and demodulates uplink data from the toolstring 116 forprocessing and storage by the surface data acquisition computer 118.

The downhole telemetry cartridge 110 contains circuitry to modulateuplink data from the tools of the toolstring 116 for transmission up thewireline cable 114 to the surface data acquisition computer 118 and todemodulate downlink commands or data from the surface data acquisitioncomputer 118 for the tools of the toolstring 116.

A more detailed schematic view of an example cable telemetry system 200is shown in FIG. 2. The cable telemetry system 200 shown includes asurface acquisition module/surface modem (DTM) 220 having a telemetryinterface module (TIM) 222, which can be located at the surface as aportion of or operatively coupled to the surface data acquisition frontend 119 (a component of surface data acquisition computer 118 of FIG.1). The front end 119 may be, for example, eWAFE™ commercially availablefrom SCHLUMBERGER™ (see: www.slb.com).

The surface data acquisition front end 119 is coupled to the wirelinecable 114, and a downhole modem (DTC) 226 (as a portion of the downholetelemetry cartridge 110 at the head of the toolstring 116 of FIG. 1).The tool string 116 includes a number of downhole tools, 230, 230′,230″, 230′″, etc. The downhole tools 230, 230′, etc., each containing arespective interface package, 232, 232′, 232″, 232′″, etc., throughwhich they are in communication with the DTC 226 via a tool bus 228. Thedownhole tools 230, 230′, 230″, 230′″, etc. may also have tool nodecontrollers 233, 233′, 233″, 233′″, etc., respectively.

The cable telemetry system 200 may handle data flows in oppositedirections (i.e., from the tools 230, 230′, etc.) via the respectivenode and the tool bus 228. The flow extends to the DTC 226 to the DTM220 over the cable 114 (“uplink”), and the reverse direction from theDTM 220 to the DTC 226 and tools 230, 230′, etc., over the same path(“downlink”). The cable telemetry system 200 provides a communicationpath from the tools, 230, 230′, etc., to the DTM 220 of the dataacquisition computer 118 so that data acquired by sensors 231, 231′,231″, 231′″, etc. of the downhole tools 230, 230′, etc. can be processedand analyzed at the surface, as well as communication between tools 230,230′, etc.

Each individual tool (230, 230′, etc.) may include a node command buffer(not shown) at the interface package 232, 232′, etc., as well as a logiccontroller of its own (not shown). The surface acquisition front-endunit 119 may also include various additional components, such as a powermodule 221, a depth and tension module 223, and a flow controllersoftware module (FEPC) 224.

The downhole telemetry cartridge 226 can include a downhole master nodecontroller 227 that may examine packets sent by each respective tool230, 230′, etc. Data communicated in either direction may be copied andbuffered at the master node controller 227, and sent to the recipient.

A surface computer 234 can store and execute a surface data dispatchermodule 236 (which may be, in an embodiment, a software data routingmodule, such as SCHLUMBERGER's™ MAXWELL™ framework). The surfacecomputer 234 can also store and execute a plurality of surfacetool-specific applications 238, 238′, 238″, 238′″, etc. that analyze anduse data obtained, respectively, by tools 230, 230′, etc.

The wireline cable 114 may be a monocable, coaxial cable, or amulti-conductor cable, such as a heptacable. Embodiments may extend towired drill pipe or in a system that uses a single insulated conductor(wire) and borehole casing as a system. In construction, the monocablemay have a single conductor with a return on the armor of the wires,while the coaxial cable can have a single conductor with a return on theserve around it isolated from the armor.

Heptacables may contain several electrical conductors, for example,seven wires. The outer armor, for example, may have a center conductorwith six conductors wound helically around the center conductor. Datamay be simultaneously transmitted on these several conductors. Thedistinct combinations of conductors used in heptacable or othermulti-conductor cables are referred to herein as “propagation modes.”

Cross-talk between several propagation modes, when in usesimultaneously, can be a source of noise in data transmission. “Far-endcross-talk” is the interference between data transmitted in apropagation mode and data transmitted in another propagation mode.Far-end cross-talk can be caused, for example, by imperfections in thesymmetry or insulation of the wireline cable, as well as circuitry thatis used for interfacing to the cable downhole and at the surface.Far-end cross-talk can impact both data rate and robustness of the datatransmission. Cross-talk may limit the available data rate andreliability. For example, cross-talk can lead to transmission failuresduring the progress of a logging job.

A heptacable can provide for various signal propagation modes, each ofwhich transmits signals on a specific combination of the sevenconductors and armor. FIGS. 3-1 through 3-3 depict end views of examplewireline heptacable with conductors 351 a-g in a cable 350. These viewshighlight which conductors are activated for T5, T6 and T7 propagationmodes, respectively. These views depict which conductors are activatedper propagation mode for the T5, T6, and T7 propagation modes. In the T5mode of FIG. 3-1, a signal is propagated on conductors 351 c and 351 f,and the return is provided on conductors 351 a and 351 d. In the T6 modeof FIG. 3-2, the signal is propagated on conductors 351 b, 351 d, and351 f, and the return is on conductors 351 a, 351 c, and 351 e. In theT7 mode of FIG. 3-3, the signal is propagated on conductor 351 g and thereturn is on conductors 351 a-351 f and on the surrounding armor 350.

Referring now to FIG. 4, uplink and downlink signals are transmitted ina multi-carrier frequency, full duplex architecture. The graph showssignal magnitude (y-axis) vs. frequency (x-axis) for a transmitteduplink and downlink spectrum allocation. The full duplex architecturetransmits an uplink message 460 and a downlink message 462simultaneously or overlapping in time. The transmission is with a firstpropagation mode being devoted to uplink messages and a secondpropagation mode being devoted to downlink messages across a commonfrequency bandwidth.

Uplink message 460 propagates in the first propagation mode during timeperiod 461. Downlink message 462 propagates in the second propagationmode during time period 463. The time period 463 overlaps in time with aportion of time period 461, but in differing frequency bands. In theexample illustrated in FIG. 4, T7 propagation mode may be used fordownlink messages, while T5 propagation mode may be used for uplinkmessages. Given that uplink data volume is greater than downlink volumedata, downlink messages overlap in time with a fraction of uplinkmessages.

In full duplex architectures simultaneously using two modes (e.g., bothT5 and T7) for data transmission in either uplink or downlinktransmission, several effects can be noted. Using two modes for uplinkresults in both modes using overlapping frequency bands. Due to theproperties of wireline cables, frequency band overlap results in somefar-end cross-talk that causes the uplink signal transmitted on one modeto be seen in the other mode's uplink receiver.

For example, the T5 downhole transmitter's signal (intended for thesurface T5 receiver) may also be detectable in the surface T7 receiverdue to far end cross-talk (FEXT). The magnitude of the FEXT dependsmostly on the construction and maintenance of heptacable symmetry. Asthere may not be a dedicated mode for downlink, it is possible there maybe another cross-talk called “ECHO” in that a receiver may see a portionof the locally transmitted signal on the same mode. As the cross-talksources are additive in nature, the maximum achievable data rate may belowered as the cross-talk sources increase in magnitude.

ECHO, in turn, may result in two effects on long cables using fullduplex architecture. On long cables (with high attenuation), thereceiver may use a high gain in order to minimize analog to digitalconverter (ADC) quantization noise. However, the gain may also amplifythe ECHO, and the receiver can be saturated. As saturation is anon-linear distortion, the signal-to-noise ratio (SNR) may be affectedacross frequencies, not just those used by the downlink. Thus, for afull duplex architecture, receiver gain may be purposefully limited toreduce the chance of saturation. However, such a measure can increasethe impact of ADC and receiver noise and, consequently, degrade SNR,resulting in lower data rates.

A second result with ECHO is that frequency-domain separation betweenuplink and downlink is possible when the shortened impulse response ofthe main channel and the ECHO channel can be simultaneously containedwithin a discrete multi-tone (DMT) cyclic prefix. The impulse-responseof the ECHO may be affected by the impedance mismatch (both in magnitudeand phase) between the transceiver and the cable's impedance. This maymake the ECHO impulse response long and difficult to shorten.

In some cases, the ECHO may be sufficient to disrupt the integrity ofthe telemetry link in full duplex architecture. However, the ECHO mayhave an impact in the full duplex architecture where the uplink anddownlink happen simultaneously and the separation is frequency based.The architecture disclosed herein, referred to as “half duplex”architecture, can avoid the effects of ECHO by separating uplink anddownlink in time. The half duplex architecture does not suffer from ECHOas there may not be simultaneous transmission of uplink and downlink.

Referring now to FIG. 5, uplink signals and downlink signals transmittedin a multi-carrier frequency, half duplex architecture graphicallyplotted as signal magnitude versus frequency are shown. The graphillustrates signal magnitude (y-axis) vs. frequency (x-axis) showingtransmitted uplink and downlink spectrum allocation. The half duplexarchitecture described herein retains the discrete multi-tone modulationtechnique but transmits uplink and downlink during separate timeintervals. The half duplex architecture described herein also inserts ablock of “guard” samples where both the uplink and downlink are quietand do not transmit any signal. The guard period gives time for the ECHOimpulse response caused by the last transmitted frame to dissipatebefore the other direction's receiver begins receiving data.

Uplink signals 570 are transmitted in both T7 mode 571 and T5 mode 573during time periods for uplink 576. Downlink messages 572 aretransmitted in T5 mode 573 during time periods for downlink 575. Timeperiods for downlink 575 and time periods for uplink 576 do not overlap,thus the signals are transmitted sequentially, not simultaneously.

A guard period 574 is interjected between uplink signals 570 anddownlink signals 572. The guard period, during which signals are nottransmitted in either direction, allows sufficient time for the residualenergy caused by the most recently transmitted frame to dissipate beforethe other direction's telemetry unit begins receiving data. A long guardinterval may affect the duty cycle (and data rate) due to the increasedtime overhead.

In embodiments employing shorter cables, the effect of ECHO may be lesssevere, and the quiet period 574 can be shortened. In an embodiment, theguard period 574 may be configurable at runtime to optimize the uplinkduty cycle. In an embodiment, the guard period 574 may be a parameterthat is configured by the surface modem and transmitted to the downholemodem during a training phase as disclosed in related and commonlyassigned application entitled “TOOLSTRING COMMUNICATION IN CABLETELEMETRY” (Attorney Docket Number IS12.2286) filed concurrentlyherewith. In an embodiment, the length of the guard period 574 may betransmitted to the other modem during training to allow for futuremodification/tuning, even if initially fixed at a particular value.

At the surface modem, the guard period 574 provides time for thedownlink ECHO to decay away before uplink frames are received. In a halfduplex system, the downlink frames represent overhead time that is notspent transmitting uplink data. Thus, to maximize the uplink data rate,the number of downlink frames per uplink/downlink transmission may beminimized. This overhead, the guard period 574, may be minimized bytransmitting as many uplink frames as possible before switching todownlink.

An embodiment may have symmetrical use of the propagation modes whereuplink and downlink transmissions are present in the propagation modes.Another embodiment may assign downlink or uplink transmissions to one ormore propagation modes depending on the data communication rates of thesystem.

When the downlink portion of the telemetry link is established, thedownhole receiver may drop samples in order to align itself with thecable delay. If the uplink frames are transmitted at a fixed interval,then an insufficient guard period may cause a portion of the receiveddownlink frame to overlap the transmitted uplink frame. The overlap maybe seen as ECHO in the downlink receiver and implies the previouslymentioned effects (clipping, inter-carrier interference, etc.)associated with ECHO and full duplex systems. A half duplex system mayhave uplink and downlink using the same frequencies. Thus, the guardperiod may be selected as longer, at least, than the cable round-tripdelay to prevent the downlink and uplink frames from overlapping.

Referring now to FIG. 6, a flow chart for a method 600 of multi-carrierfrequency, half duplex cable telemetry. The method 600 begins withgenerating 680 downlink messages in a first propagation mode in asurface telemetry unit. In an embodiment, the cable includes a wirelineheptacable. In an embodiment, the first propagation mode includes, forexample, T5 propagation mode. FIG. 7 depicts end views of sevendifferent propagation modes T1 through T7 that may be applied inwireline communication along a heptacable.

Referring still to FIG. 6, the method continues with generating 682uplink messages in the first propagation mode and a second propagationmode in a downhole telemetry unit. In an embodiment, the secondpropagation mode includes, for example, T7 propagation mode.

The method continues with transmitting 684 the downlink messages and theuplink messages over a cable operatively coupling the surface telemetryunit and the downhole telemetry unit sequentially in a plurality of timeperiods across a single frequency bandwidth.

The method continues with separating 686 each message from thesubsequent message by a quiet time sample during which no signal istransmitted. In some embodiments, the length of the quiet time sample isdetermined based on the length of the cable. In some embodiments, thelength of the quiet time sample is determined based on at least one of atime interval sufficient for a cross-talk ECHO to dissipate, and atravel latency time sufficient to avoid overlap of the downlink messagesand the uplink messages. In an embodiment, the length of the quiet timesample includes a variable user input.

Multi-Carrier Frequency, Half Duplex Cable Telemetry Explanation

Wireline operations may be performed on a number of different cabletypes based on the number of conductors in the cable. The monocable mayhave a single conductor with a return on the armor wires, while thecoaxial cable can have a single conductor with a return on the servearound it isolated from the armor. The heptacable may have, for example,seven conductors and armor.

Enhanced Digital Telemetry System (EDTS) version 2.0 (commerciallyavailable from SCHLUMBERGER TECHNOLOGY CORPORATION™ see: www.slb.com)cable telemetry is intended to work with the heptacable-constructioncable. The heptacable may have seven natural modes. EDTS 2.0 cabletelemetry uses two of these modes: T5 and T7. T5 mode is a differentialmode that uses wires 2, 3, 5 and 6, and has immunity to common modenoise. T7 uses wire 7, a return on the remaining wires and the armor,and, hence, behaves as coaxial cable. These two modes may possess theselect telemetry channel characteristics in earlier telemetry systems,and may be used in wireline telemetry systems. FIG. 7 shows an end viewof seven different propagation modes T1 through T7 applied in wirelinecommunication along a heptacable.

A purpose of cable link telemetry (as shown in FIGS. 8 and 9) may be toenable data reception from the tools of the toolstring and to enablesending commands to the tools via the wireline heptacable. FIG. 8depicts data transfer between a MAXWELL™ tool application, an upholemodem, a downhole modem and a tool. The communication between theMAXWELL™ tool application and an uphole modem and the communicationbetween the downhole modem and the tool involve uplink data andcommands. The communication between the uphole modem and the downholemodem involves modem signals (cable telemetry). The cable telemetry mayprovide delivery from uphole to downhole modem only. The EDTS 2.0components may be used to provide delivery from MAXWELL™ toolapplication to tool.

FIG. 9 shows data transfer between a surface modem (e.g., with MAXWELL™tool applications) and a downhole toolstring. The transfer involvesdownlink via logging cable from a cable telemetry to a toolstring with adownhole modem m, tools 1 and 2, and uplink back thereto.

Cable telemetry is configured to avoid altering data or commands. EDTS2.0 cable telemetry is built in the uphole and downhole modems. Thisuses a multi-carrier modulation technology called DMT. This technologymay also be used in other telemetry systems, such as EDTS (a predecessorof EDTS 2.0 commercially available from SCHLUMBERGER TECHNOLOGYCORPORATION™ see: www.slb.com). While EDTS cable telemetry uses a singleuplink channel on T5 and a single downlink channel on T7, EDTS 2.0 mayuse both T5 and T7. Using both T5 and T7 may provide uplink datatransmission at nearly double the data rate, and may use T5 for downlinktransmission.

Due to the properties of wireline cables, there may be some FEXT thatcauses the uplink signal transmitted on one mode to be seen in the othermode's uplink receiver. For example, the T5 downhole transmitter'ssignal (intended for the surface T5 receiver) may also be detectable inthe surface T7 receiver due to FEXT. The magnitude of the FEXT maydepend on the construction and maintenance of heptacable symmetry. Asthere may be no dedicated mode for downlink, there may be another FEXTcalled “ECHO” in that a receiver may see a portion of the locallytransmitted signal on the same mode. As the FEXT source may be additivein nature, the maximum achievable data rate may be lowered as thecross-talk sources increase in magnitude.

As shown in FIG. 10 a cumulative signal (i.e. Actual signal seen atreceiver) may be generated from a combination of a receiver signal (RX),cross-talk (ECHO), near end cross-talk (NEXT), and noise effects added.

FIG. 11 shows a schematic of cumulative cross-talk (ECHO) sources on awireline heptacable (i.e. transmission cable). The heptacable mayinclude NEXT, FEXT and other components. The ECHOS extend from TX A andB to hybrids and back to RX C and D on one side, and ECHOS extend fromTX C and D to hybrids and back to RX A and RX B on another side of theheptacable.

The ECHOs may result in two effects on long cables for the full duplexarchitecture. On long cables (e.g., with high attenuation), the receivermay use a high gain in order to minimize ADC quantization noise.However, the gain may also amplify the ECHO, and the receiver can besaturated. As saturation is a non-linear distortion, it may impact theSNR on various frequencies, not just those used by the downlink. So witha full duplex architecture, receiver gain may have to be limited toreduce the chance of saturation. However, such a measure may increasethe impact of ADC and receiver noise and consequently degrade SNR, whichmay result in lower data rates.

A second effect with ECHO is that frequency-domain separation betweenuplink and downlink may be possible when the shortened impulse responseof the main channel and the ECHO channel can be simultaneously containedwithin a DMT cyclic prefix. Additionally, the impulse response of theECHO can be affected by the impedance mismatch (both in magnitude andphase) between the transceiver and the cable's impedance. This may makethe ECHO impulse response long and difficult to shorten.

The effect of ECHO can be severe in full duplex architectures. The levelof ECHO may be as much as 50 dB more than NEXT on a telemetry interfacemodule (e.g., eWAFE™), and can have a negative impact on the uplink datarate for full duplex architecture. In some cases, the ECHO could besufficient to disrupt the integrity of the telemetry link.

ECHO is a property of the physical interfaces, such as the hybrid andits impedance matching to the cable, as well as the relative size of thesignal to the ECHO which may depend on the cable characteristics. TheECHO may have an impact in the full duplex architecture where the uplinkand downlink happen simultaneously and the separation is frequencybased. Thus, the half duplex architecture may avoid the effects of ECHOby separating uplink and downlink in time. The half duplex architecturemay not suffer from ECHO as there is no simultaneous transmission ofuplink and downlink.

To overcome the effects detailed above, the following physical layerarchitecture may support, for example, about 150 KHz, 200 KHz, and 250KHz bandwidths, have an uplink/downlink structure, about 36 uplinkframes per 1 downlink frame at 150 KHz, 200 KHz, 250 KHz bandwidths, andabout 100 fixed samples of quiet time between uplink/downlink at 150KHz, 200 KHz, 250 KHz bandwidths. The quiet time value may betransmitted to other modes during training to allow for futuremodification. The architecture may also allow for single-mode operationon T5 or dual uplink mode (T5+T7) operation. The architecture may alsoallow variable downlink bit allocation (150 KHz: ˜900 bits total, ˜600data bits, ˜300 maintenance bits), thus allowing for data rates to bemet at 150 KHz bandwidth and above.

FIG. 12 shows a time-domain diagram of uplink and downlink frames in ahalf duplex architecture. This time-domain diagram depicts T5 and T7schedules. The T5 schedule includes 1 T5 downlink (DL) frame, 50 T5quite samples, 36 T5 uplink (UL) frames and 50 T5 quiet samples. The T7schedule includes a quite frame, 50 T5 quite samples, 36 T7 uplinkframes and 50 T7 quiet samples. FIGS. 13-1 and 13-2 show oscilloscopecaptures for a time domain diagram. These figures showing uplink framesand downlink frame therebetween. FIG. 13-2 is a zoomed-in version ofFIG. 13-1.

The half duplex architecture retains the discrete multi-tone modulationtechnique, but transmits uplink and downlink during separate timeintervals. It also inserts a block of “guard” or “quiet” samples whereboth the uplink and downlink do not transmit any signal. This periodgives time for the (shortened) ECHO impulse response caused by the lasttransmitted frame to dissipate before the other direction's receiverbegins receiving data.

Sampling timing synchronization may be performed for the systems herein.Example sampling timing synchronizations that may be used are describedin U.S. Pat. No. 6,778,622. In the half duplex system of the presentdisclosure, there are two ways sampling timing can be maintained. Thesurface FPGA sample frequency offset may change value slowly (forexample, in an embodiment, each few minutes). This mechanism may trackwithout updates for a few frames using the previously known offset.Since per-frame clock shifts by design occur with probability less thanone half (50/100) and the downlink+quiet interval may be short (in anexample embodiment, less than 2 frames), the per-frame clock shifts canaccount for the lack of adjustment during this interval. The first fewuplink frames may experience a slightly larger shift, but the effect ofthis shift can be remedied through digital rotations.

Linear regression estimated during the uplink interval could be used toextrapolate the pilot angle through the quiet & downlink periods. Thismay permit the clock cycle shift to be applied during the downlink &quiet intervals using the predicted pilot tone phase. As EDTS mayperform linear regression over hundreds of frames (e.g., extrapolationover one or two frames can be performed). By the next uplink frame, theextrapolation would predict the true pilot tone phase taking intoaccount any clock cycle adjustments.

The system may also be used with variable quiet time. At first glance,the quiet period could be an entire frame's worth of samples, as theECHO impulse response may die off exponentially. However, such a longguard interval may lower the duty cycle (and data rate) due to theincreased time overhead. Furthermore, the quiet period may be longenough for the shortened (post-TEQ) ECHO impulse response to dissipate.Although the time domain equivalent (TEQ) may or may not explicitly tryto shorten the ECHO impulse response.

FIGS. 14-1 and 14-2 are graphs showing a shortened ECHO impulse responseafter TEQ, and a corresponding un-shortened ECHO impulse response beforeTEQ, respectively. Each graph depicts value (y-axis) versus samplenumber (x-axis). These figures show the ECHO impulse response on 36 kfeet (ft) (10.97 km) 7-48 A heptacable for 200 KHz bandwidth. Althoughclipping may occur in the un-shortened impulse response, the duration inwhich the un-shortened impulse response has large enough amplitude tocause clipping can be neglected.

Using the above observations, in an example embodiment, 50 samples ofquiet time may be sufficient at 150 KHz bandwidth on 36 k ft (10.97 km)of 7-48 A heptacable. This amount of quiet time may not be sufficientfor 200 KHz and 250 KHz. On shorter cables the effect of ECHO may beless severe and the quiet interval may be shortened. Thus it may beuseful to have the quiet portion configurable at runtime to optimize theuplink duty cycle. It could be a parameter that is configured by thesurface modem and transmitted to the downhole modem during training. Thequiet interval length may be transmitted to the other modem duringtraining to allow for future modification/tuning (even if it isinitially fixed at a particular value, such as 100 samples).

The length of the guard interval is a design parameter that may be usedfor half duplex architecture. The guard interval provides time for thedownlink ECHO to decay away before the uplink frames are received. Thus,the guard interval may be longer than the (shortened) ECHO impulseresponse (at the surface).

The guard interval may provide a “cushion” at the downhole modem,between the downlink receiver receiving downlink frames and the uplinktransmitter transmitting uplink frames. In EDTS, the downlink frameboundaries may be aligned to the uplink frame boundaries on the surfacemodem. This alignment may result from the downhole (uplink) transmitterinitiating the telemetry link at power up. The same alignment may carryover into EDTS 2.0. When the downlink portion of the telemetry link isestablished, the downhole receiver may have to drop samples in order toalign itself with the cable delay. If the uplink frames are transmittedat a fixed interval, then an insufficient guard period may cause aportion of the received downlink frame to overlap the transmitted uplinkframe.

The overlap may be seen as ECHO in the downlink receiver and implies thepreviously mentioned issues (e.g., inter-carrier interference, etc.)associated with ECHO and full duplex systems. Moreover a half duplexsystem may have uplink and downlink using the same frequencies. Thus,the guard period may be selected as longer than the cable round-tripdelay to prevent the downlink and uplink frames from overlapping.

FIG. 15 graphically shows an effect of delay on guard interval length ofthe frames. This figure includes three graphs of T5 downhole(conceptual), T5 surface, and T5 downhole (actual) (y-axis) versus time(x-axis), respectively. Each figure also includes an uplink and adownlink with two guards and downlink therebetween. The downlink ispositioned between the two guards. Cable delays extend between theuplinks of the top two graphs. Another cable delay extends from thedownlink of the middle graph to the downlink of the bottom graph.

Table 1 provides example (one-way) cable latencies measured for 150 KHzbandwidth:

TABLE 1 T5 Delay T7 Delay Cable (Samples) (Samples)  7 k ft P 4 4 21 kft A 13 11 31 k ft NT 23 21 31 NT + 7p 27 25

In an example embodiment, a 50 sample guard interval may work for up toabout 36 k ft (10.97 km) 7-48 A cable, the round-trip delay of such acable (at about 150 KHz) is approximately 45 samples (36 k ft (10.97km)/21 k ft (6.4 km)×13 samples×2). At 200 KHz the round-trip delay isabout 60 samples. If a 3 tap TEQ is used, the cyclic prefix of the firstuplink frame provides an amount of additional guard time (e.g., about 14samples). In short, the guard interval may be set to a value greaterthan 50 samples for 200 KHz (or about 250 KHz) bandwidth operation, forexample on longer cables. At least 100 samples may allow for half duplexoperation on cables up to about 80 k ft (24.38 km) at about 150 KHz and48 k ft (14.63 km) at 250 KHz. Furthermore, a 100 sample guard intervalmay not decrease the duty cycle.

In terms of the physical layer, the following tasks may be performed ateach uplink frame: update the frequency-domain equalizer (FEQ), updatethe mean-squared noise (MSN) measurement (for SNR), generate bitswapping tone list for the uplink channel (each ˜2 seconds), andoptionally bit swapping tone list for the downlink channel.

In the half duplex architecture of the present disclosure, theseoperations could be split across data frames and/or performed during thequiet and downlink intervals. The SYNC frame may not be called for inEDTS 2.0 and may be reduced in the half duplex architecture.

For the downlink, half duplex EDTS 2.0 may not call for bit-swapping. Inan embodiment, the minimum downlink margin may be about more than 20 dBon about 36 k ft (10.97 km) cable across various tones on a DMT testbed.Optionally, downlink bit-swapping could be implemented using the firstor last uplink frame as a reference.

Downhole on-board-programming (OBP) mode may also be used. Where thedownlink data rate may be about 8-9 kbps, few (if any) downlink commandsmay be sent after a toolstring is initialized. In some circumstances, alarge amount of downlink data may be sent. For example, downhole EPROMprogramming and/or downhole firmware upgrade operations call for a largeamount of data to be transferred downhole. In an embodiment, theseoperations may be performed in a shop or lab, rather than during a fieldjob and may call for a minimal uplink data rate. For these operations,the number of uplink frames between downlinks could be reduced in orderto support faster downlink data rates.

Implementation options, such as those in Table 2, may be used to insurecompatibility with previous systems. Uplink data rates represent thetotal sum of T5 and T7 data rates.

TABLE 2 Uplink Data (As high as possible) Uplink Maintenance >=4.48 kbpsDownlink Data >=15 Hz, >=8.96 kbps Downlink Maintenance >=8.235Hz, >=4.48 kbps

In 150 KHz system, a DMT frame of 256 carriers and 16 sample cyclicprefix may result in a frame rate of 568.18 Hz. In a half duplex system,the quiet period is an overhead that decreases the uplink and downlinkthroughput. This overhead may be minimized by transmitting as manyuplink frames as possible before switching to downlink. The downlinklatency requirement (e.g., greater than about 15 Hz) constrains themaximum number of uplink frames between downlinks. Since 568.18 Hz/15Hz=37.87, there may be a downlink frame, for example, at least each 37frames. In other words, there may be no more than 36 frames per 1downlink frame (assuming that an entire downlink packet could betransmitted in one downlink frame, not including time for the quietinterval). Similarly, there may be a downlink frame at least each 50frames for 200 KHz bandwidth, 63 frames for 250 KHz bandwidth.

In a half duplex system, the downlink frames represent time that is notspent transmitting uplink data. The uplink data rate may be maximized byminimizing the number of downlink frames per uplink/downlinktransmission. This may cause relative higher number of bits to beallocated to the downlink frames.

About 1500 total bits may be called for the downlink to support the dataand maintenance channels at a 15 Hz transmission rate. As there arefewer than 256 carriers per frame, using a fixed bit allocation (forvarious cables) may not be a viable option. For example, putting 4 bitson 200 tones may result in margins in the lower tones (where SNR isgood) and a lower margin on the higher tones where the SNR is poor. Inan embodiment, the SNR may change by as much as 20 dB from the peak tothe minimum. Such an allocation may also be prone to narrowbandinterference generated by the power systems, etc.

For half duplex, the downlink should dynamically allocate bits based onthe measured SNR. The per tone gains may not be varied (e.g. “finegains”), but the number of bits may be varied according to SNR. Such anallocation may improve the minimum downlink margin across various tones.

When considering half duplex systems, the “duty cycle” (portion of timethat the uplink is actively transmitting data) may be computed. Dutycycle can then be used to compare the efficiency of a half duplex systemversus that of an EDTS (which transmits data 68/69=98.6% of the time).The number of bits per frame to sustain a particular data rate and tocalculate the downlink latency (due to quiet portions) may also becomputed.

The half duplex duty cycle may be represented by Equations 1 and 2:

$\begin{matrix}{{{UplinkDutyCycle}_{HalfDuplex} = \frac{\left( {\# {{UplinkFrames} \cdot {UplinkFrameTime}}} \right)}{T}}{{UplinkDutyCycle}_{HalfDuplex} = \frac{\left( {\# {{UplinkFrames} \cdot {UplinkFrameTime}}} \right)}{T}}} & {{Equation}\mspace{14mu} (1)} \\{T = {\left( {\# {{UplinkFrames} \cdot {UplinkFrameDuration}}} \right) + \left( {\# {{DownlinkFrames} \cdot {DownlinkFrameDuration}}} \right) + \left( {\# {{QuietSamples} \cdot {QuietSampleDuration}}} \right)}} & {{Equation}\mspace{14mu} (2)}\end{matrix}$

where T is the total time.

If both uplink and downlink use the same FFT size (512 samples) and thesame cyclic prefix (16 samples), then the frame length is 528 samplesand the duty cycle and consequent data rate become the following inEquations 3 and 4:

$\begin{matrix}{{{UplinkDutyCycle}_{HalfDuplex} = \frac{\left( {\# {UplinkFrames}} \right)}{\begin{matrix}{\left( {\# {UplinkFrames}} \right) + \left( {\# {DownlinkFrames}} \right) +} \\{2 \cdot \left( {\# {{QuietSamples}/528}} \right)}\end{matrix}}}{{UplinkDataRate}_{HalfDuplex} = {{UplinkDutyCycle}_{HalfDuplex}*{UplinkBitsPerFrame}}}} & {{Equation}\mspace{14mu} (3)} \\{{{DownlinkFrequency}_{HalfDuplex} = {{({RawFrameRate})\; \frac{\left( {1{DownlinkFrame}} \right)}{\begin{matrix}{\left( {\# {UplinkFrames}} \right) + \left( {1{DownlinkFrame}} \right) +} \\\left( {2*{{QuietSamples}/528}} \right)\end{matrix}}} = {({RawFrameRate})\; \frac{1}{\begin{matrix}{\left( {\# {UplinkFrames}} \right) + 1 +} \\\left( {2*{{QuietSamples}/528}} \right)\end{matrix}}}}}\mspace{20mu} {{RawFrameRate} = {\left( {{Bandwidth}/2} \right)/\left( {528\mspace{14mu} {Samples}} \right)}}} & {{Equation}\mspace{14mu} (4)}\end{matrix}$

Table 3 below show various parameters for various bandwidths:

TABLE 3 Frame Uplink Downlink Downlink Downlink Downlink Bandwidth RateUplink Downlink Quiet Duty Duty Frequency Data Maintenance (KHz) (Hz)Frames Frames Samples Cycle % Cycle % (Hz) Bits/Frame Bits/Frame 150568.18 36 1 0 97.3 2.7 15.36 583 292 150 568.18 36 1 50 96.8 2.7 15.28586 293 150 568.18 36 1 100 96.3 2.7 15.20 589 295 150 568.18 36 1 25094.9 2.6 14.97 598 299 150 568.18 36 1 528 92.3 2.6 14.57 615 308 200757.58 36 1 0 97.3 2.7 20.48 438 219 200 757.58 36 1 50 96.8 2.7 20.37440 220 200 757.58 36 1 100 96.3 2.7 20.27 442 221 200 757.58 36 1 25094.9 2.6 19.96 449 224 200 757.58 36 1 528 92.3 2.6 19.43 461 231 250946.97 36 1 0 97.3 2.7 25.59 350 175 250 946.97 36 1 50 96.8 2.7 25.46352 176 250 946.97 36 1 100 96.3 2.7 25.33 354 177 250 946.97 36 1 25094.9 2.6 24.96 359 180 250 946.97 36 1 528 92.3 2.6 24.28 369 185 Uplinkinterval increased to maintain same downlink data rate as 150 KHz: 200757.58 49 1 0 98.0 2.0 15.15 591 296 200 757.58 49 1 50 97.6 2.0 15.09594 297 200 757.58 49 1 100 97.3 2.0 15.04 596 298The downlink duty cycle can be calculated in a similar manner. It isalso related to the frequency at which downlink transmissions are sent.For sending one downlink frame per uplink, the following is found:

In an embodiment, downlink packets may arrive continuously at a rate notless than about 15 Hz. If one downlink frame is transmitted betweenuplinks at 150 KHz bandwidth, then no more than about 36 uplink framesmay be transmitted by the downlink frame as shown in Table 3 above. Thelength of the quiet interval may have a minor effect on the effectivedownlink frequency.

In an embodiment, if an entire downlink maintenance packet (maximum of160 bits) is transmitted at 15 Hz (in a single downlink frame), thenless memory may be used for the downhole uplink superpacket buffers andthe master-slave clock synchronization may be improved over EDTS. As the15 Hz rate is used for the downlink data channel (and it is possible toallocate more than about 160 bits/frame for the downlink maintenancechannel), the changes in EDTS 2.0 may be obtained at low cost.

The calculations involving the number of uplink frames per downlinkframe consider the minimum number for 150 KHz bandwidth. In anembodiment, the downlink data and maintenance channel may increase inboth rate and frequency due to the faster frame rate achieved at 200 KHzbandwidth. For half duplex EDTS 2.0 at 200 KHz bandwidth, the downlinkdata rate and frequency may not increase. The downlink maintenancechannel frequency also may not increase since the uplink superpacketrate is not dependent on bandwidth. To maintain 15 Hz downlink rate witha frame rate of 757.58 Hz (200 KHz bandwidth), there could be as many as49 uplink frames per downlink frame (757.58/15=50.503). With no quietperiod, the maximum 150 KHz duty cycle is 36/37=97.3%. The maximum 200KHz duty cycle would be 49/50=98%. The difference in duty cycles mayaffect implementation. For example (not including quiet time), a systemthat achieves 3 Mbps from a 97.3% duty cycle may achieve 3.022 Mbps witha 98% duty cycle.

FIG. 16 shows a process for managing superpacket data flow. This figuredepicts uplink and downlink signals using a T5 and T7 cable. In anembodiment, the EFTB 2.0 is designed to be more independent from themodem process compared to the EFTB in EDTS. At the top of FIG. 16, foursignals can be seen. From left to right, an outgoing uplink signal outto, for example, T5 cable propagation mode from an uplink digital toanalog converter, an incoming downlink signal from, for example, T5cable propagation mode, into a downlink analog to digital converter, anincoming downlink signal from, for example, a T7 cable propagation modeto a downlink analog to digital converter, and an outgoing uplink signalto, for example, a T7 cable propagation mode, from an uplink digital toanalog converter.

In specific detail, the downlink signals will be discussed first (themiddle incoming signals at the top of FIG. 16 respectively). The T5downlink signal (2^(nd) from the left at the top of FIG. 16) passes fromthe downlink analog to digital converter to a downlink process (ininternal SRAM) of the T5 uplink MCM via input SP0RX (DMA0), to bebuffered at internal SRAM in the various buffers: Receiver SymbolBuffer, Receiver Superpacket Buffer, and Receiver Superpacket TransferBuffer, respectively. The T5 downlink signal is then passed to thedownlink path switch on the EFTB+MCM via input SP1TX (DMA3). On theEFTB+MCM, the signal is passed via input SP1RX to buffers downlinksuperpacket receiver buffer, to downlink packet buffer, to EFTB downlinkpacket transmitter buffer, to downlink process, and out to EFTB on inputSP0TX (DMA2). The data can also be buffered in external SRAM 3×4M atdownlink superpacket buffer temporarily, and then passed to downlinksuperpacket receiver buffer, downlink packet buffer, and so on.

The T7 downlink signal (3^(rd) from the left at the top of FIG. 16) ispassed from the downlink analog to digital converter to the T7 downlinkMCM on the input SP0RX (DMA0) to downlink process (in internal SRAM) ofthe T7 downlink MCM. The signal is buffered in the various buffers ofthe internal SRAM of the T7 downlink MCM: Receiver Symbol Buffer,Receiver Superpacket Buffer, and Receiver Superpacket Transfer Buffer,before passing to the downlink path switch on the EFTB+MCM via inputSP1TX (DMA3). Once on the EFTB+MCM, the T7 downlink signal can passalong the same data pathway that the T5 downlink signal follows throughthe EFTB+MCM.

An uplink signal enters the EFTB+MCM from the EFTB at the bottom leftinput on FIG. 16, passing to a digital signal processor DSP via inputSP0RX (DMAO). The uplink signal then can pass through various buffers:EIP Queue Message Buffer, Uplink Controller Buffer, and Uplink PacketBuffer. The data signal is then passed to the internal superpackettransmitter buffer (and optionally, divided into headers and trailerportions of the signal, buffered at uplink superpacket headers bufferand uplink superpacket trailers buffer). The signal is then passed tothe uplink process of the DSP of the EFTB+MCM, and passed to the SProuter of the Field Programmable Gate Array (FPGA) of the EFTB+MCM thatdetermines whether the uplink is to be transferred via T5 propagationmode or T7 propagation mode. Uplink signals determined to be transmittedusing T5 propagation mode are passed to the T5 uplink MCM, and via inputSP1RX (DMA1) passed to the internal superpacket buffer in internal SRAMof the T5 uplink MCM. The signal can be buffered in external SRAM atexternal memory superpacket list nodes temporarily. The signal thenpasses to the transmit buffer 209×32 bits, to the internal SRAM uplinkprocess, before passing to the output via input SP0TX (DMA2) to theuplink digital to analog converter at top right of FIG. 16.

Uplink signals determined to be transmitted using T7 propagation modeare passed to the T5 uplink MCM, and via input SP1RX (DMA1) passed tothe internal superpacket buffer in internal SRAM of the T7 uplink MCM.The signal can be buffered in external SRAM at external memorysuperpacket list nodes temporarily. The signal then passes to thetransmit buffer, to the internal SRAM uplink process, before passing tothe output via input SP0TX (DMA2) to the uplink digital to analogconverter at top left of FIG. 16.

Data flow may be uplinked (UL SP Data Flow) using a dual uplink EDTS2.0. The uplink superpackets at about 4 Mbps may be generated in theEFTB 2.0 DSP at a rate of 125 Hz (or about twice as much as the EFTBrate (62.5 Hz)). These packets may be routed (based on bufferavailability) to the T5 and T7 DSPs resulting in each FEDSP receivingthe overall superpackets rate at approximately 62.5 Hz. Since each ofthe uplink DSPs operates with (roughly) the same data rate as standardEDTS, the acknowledgement of previously transmitted superpackets may bemanaged by supporting a downlink with double the 8.23 Hz acknowledgementrate of EDTS.

Data flow may be uplinked (UL SP Data Flow) using single mode EDTS 2.0.The UL superpackets at about 2 Mbps may be generated in the EFTB 2.0 DSPat a rate of 62.5 Hz. These packets may be sent to the T5 DSP. Sincethis DSP operates with the same data rate as EDTS, the acknowledgementof previously transmitted SP may be managed by supporting a DL at leastthe same 8.23 Hz acknowledgement rate of EDTS.

Data flow may be downlinked (DL SP Data Flow). In half duplex EDTS 2.0,a mode (T5) is used for downlink. A single-mode downlink transmissionpath may be supported either on the T5 or T7 DSP where the T5 mode wouldbe selected as the default due to robustness against the common-modenoise.

Although a few example embodiments have been described in detail above,those skilled in the art will readily appreciate that many modificationsare possible in the example embodiments without materially departingfrom this disclosure. Accordingly, such modifications are intended to beincluded within the scope of this disclosure as defined in the followingclaims. In the claims, means-plus-function clauses are intended to coverthe structures described herein as performing the recited function andnot simply structural equivalents, but also equivalent structures. Thus,although a nail and a screw may not be structural equivalents in that anail employs a cylindrical surface to secure wooden parts together,whereas a screw employs a helical surface, in the environment offastening wooden parts, a nail and a screw may be equivalent structures.It is the express intention of the applicant not to invoke 35 U.S.C.§112, paragraph 6 for any limitations of any of the claims herein,except for those in which the claim expressly uses the words “means for”together with an associated function.

What is claimed is:
 1. A method for multiple carrier frequency, halfduplex cable telemetry for a wellsite, comprising: generating a firsttype of bi-directional message in a first propagation mode; generating asecond type of bi-directional message in the first propagation mode andin a second propagation mode; transmitting the first and second types ofbi-directional message over a cable operatively coupling a surface modemand a downhole modem sequentially in a plurality of time periods acrossa single frequency bandwidth; and separating each of the first andsecond types of bi-directional messages from a most subsequentlytransmitted one of the first and second type of bi-directional messageby a quiet time sample during which no message is transmitted.
 2. Themethod according to claim 1, wherein the cable comprises one of aheptacable, a monocable, a coaxial cable, a wired drill pipe conductor,and a slickline.
 3. The method according to claim 2, wherein the firstpropagation mode comprises a T5 propagation mode.
 4. The methodaccording to claim 2, wherein the second propagation mode comprises a T7propagation mode.
 5. The method according to claim 1, wherein the firsttype of bi-directional message comprises downlink and the second type ofbi-directional message comprises uplink.
 6. The method according toclaim 1, wherein a length of the quiet time sample is determined basedon one or more of: a length of the cable, a time interval sufficient fora cross-talk ECHO to dissipate, a travel latency time sufficient toavoid overlap of the first and second types of bi-directional message,and a variable user input.
 7. A system for multiple carrier frequency,half duplex cable telemetry for a wellsite, comprising: a surface modemthat generates one or more downlink messages in a first propagationmode; one or more downhole tools that obtain measurements relating to atleast one of borehole characteristics and formation characteristics, theone or more downhole tools coupled by a toolbus to a downhole modem, thedownhole modem generating one or more uplink messages in the firstpropagation mode and in a second propagation mode; a cable electricallycoupling the surface modem and the downhole modem; wherein the downlinkand uplink messages are transmitted over the cable sequentially in aplurality of time periods across a single frequency bandwidth, each ofthe downlink and uplink messages is separated from a most subsequentlytransmitted one of the downlink and uplink messages by a quiet timesample during which no message is transmitted.
 8. The cable telemetrysystem according to claim 7, wherein the cable comprises one of aheptacable, a monocable, a coaxial cable, a wired drill pipe conductor,and a slickline.
 9. The cable telemetry system according to claim 8,wherein the first propagation mode comprises T5 propagation mode; andwherein the second propagation mode comprises T7 propagation mode. 10.The cable telemetry system according to claim 7, wherein a length of thequiet time sample is determined based on one or more of: a length of thecable, a time interval sufficient for a cross-talk ECHO to dissipate, atravel latency time sufficient to avoid overlap of two types ofbi-directional message, and a variable user input.
 11. A system formultiple carrier frequency, half duplex cable telemetry for a wellsite,comprising: a surface acquisition unit operatively coupling to a surfacemodem that generates one or more downlink messages in a firstpropagation mode; a downhole modem that generates one or more uplinkmessages in the first propagation mode and in a second propagation mode;a downhole toolstring comprising one or more downhole sensing tools thatobtain measurements relating to at least one of borehole characteristicsand formation characteristics, the downhole toolstring operativelycoupled to the downhole modem via a toolbus; a cable electricallycoupling the surface modem and the downhole modem; wherein the downlinkand uplink messages are transmitted sequentially in a plurality of timeperiods across a single frequency bandwidth, each of the downlink anduplink messages is separated from a most subsequently transmitted one ofthe downlink and uplink messages by a quiet time sample during which nomessage is transmitted.
 12. The wellsite system according to claim 11,wherein a length of the quiet time sample is determined based on one ormore of: a length of the cable, a time interval sufficient for across-talk ECHO to dissipate, a travel latency time sufficient to avoidoverlap of the two types of bi-directional message, and a variable userinput.